Many surgical devices and materials must be sterilized prior to use for the health and safety of patients and hospital staff. Sterilization may be divided into high- and low-temperature sterilization. Generally, high-temperature sterilization is preferred since it is significantly faster than low-temperature sterilization. High-temperature sterilization involves exposing the articles to be sterilized to steam at temperatures ranging from about 250 to about 270.degree. F. in an air-tight chamber. The process can be completed usually in less than about 2 hours. However, some articles, such as plastic articles and electrical components, cannot withstand such high temperatures and require low-temperature sterilization. Low-temperature sterilization is the focus of this invention.
Typically, low-temperature sterilization involves the use of chemical sterilants at temperatures from about 100 to about 200.degree. F. Common chemical sterilants include, for example, EO, formaldehyde, hydrogen peroxide, chlorine dioxide, and ozone. In medical applications, EO is the most widely used sterilant. Standards for EO sterilization are set forth in Good Hospital Practice: Ethylene Oxide Sterilization and Sterility Assurance ANSI/AAMI ST41-1992.
Low-temperature sterilization is usually a two-step process performed in an air-tight chamber. In the first step (the sterilization step), the articles having been cleaned and wrapped in gas permeable bags are placed in the chamber. Air is then evacuated from the chamber by pulling a vacuum and perhaps by displacing the air with steam. In processes using EO as the sterilant, it is preferable to inject steam into the chamber to achieve a relative humidity that ranges preferably from about 30% to about 70%. Such humidities are found to maximize the sterilizing effectiveness of the EO sterilant which is introduced into the chamber after the desired relative humidity is achieved. After a period of time sufficient for the sterilant to permeate the wrapping and reach the interstices of the article, the sterilant and steam are evacuated from the chamber.
In the second step of the process (the aeration step), the articles are aerated to remove sterilant residues. Removing such residues is particularly important in the case of toxic sterilants, such as EO. Typical aeration processes include air washes, continuous aeration, and a combination of the two. An air wash is a batch process and usually comprises evacuating the chamber for a relatively short period, for example, 12 minutes, and then introducing air at atmospheric pressure or higher into the chamber. This cycle is repeated any number of times until the desired removal of sterilant is achieved. Continuous aeration typically involves introducing air through an inlet at one side of the chamber and then drawing it out through an outlet on the other side of the chamber by applying a slight vacuum to the outlet. Frequently, the two approaches are combined. For example, a common approach involves performing air washes and then an aeration cycle.
Low-temperature sterilization is time-consuming. Although the sterilization step can be done in less than 3 hours, the aeration step typically requires from about 8 to about 10 hours. The time between when an article is sent for sterilization and when it is returned is called "turnaround time."
There is a need to reduce turnaround time. This need stems from cost cutting pressures applied to hospitals by government and insurance companies. Since articles being sterilized are not available for use, an inventory of articles must be kept on hand to accommodate the turnaround time. Such inventory can be expensive, often times costing the hospital millions of dollars. Consequently, a great deal of effort has been directed at reducing turnaround time, particularly the aeration step which represents the majority of the turnaround time.
Recent efforts have been successful at reducing turnaround time, but they tend to create other problems which are severe enough to restrict their implementation. For example, one prior art effort involves using sterilants other then EO, such as vapor phase hydrogen peroxide or peracetic acid, and smaller sterilization chambers. Although these sterilization chambers can turnaround articles more quickly, their small capacity limits their throughput. Additionally, the alternative chemical sterilants used in these sterilizers are not as versatile as EO.
Another prior art effort to reduce aeration time involves sorting devices by ease of aeration. For example, some materials tend to be less prone to sterilant residue and/or the sterilant residue can be removed more readily. Others materials can withstand higher temperatures. Every 8.degree. F. increase in temperature can reduce sterilizing and aeration times by 50%. Therefore, by sorting articles according to their ability to withstand either higher temperatures or to retain low sterilant residue, they can be put on a "fast track" and avoid being grouped with other articles which require the use of lower temperatures or longer aeration times. Such an approach, however, is labor intensive and complicated since different operating conditions must be observed constantly. Additionally, the sorting approach runs the risk of articles being damaged or inadequately sterilized due to errors in grouping.
Yet another approach to the problem involves injecting supersaturated steam with the sterilant as described in U.S. Pat. No. 4,770,851. The supersaturated steam condenses on the articles and their wrapping. The condensed steam acts as a removal agent for the sterilant by condensing in the interstices of the article and then evaporating to carry the sterilant away. This approach, however, requires costly modification to existing systems. Additionally, the condensing of steam on articles and their wrapping can result in the formation of undesirable "wet packs." Wet packs compromise the sterilization process by providing a medium (water) in which bacteria live and migrate. Contact with water may also damage certain articles such as electronic components. Thus, this approach reduces time, but can increase the risk of inadequate sterilization of and damage to the articles.
Therefore, a need exists for a less time-consuming aeration procedure that can be practiced on all articles that require low-temperature sterilization without compromising sterilization or damaging the articles. The present invention fulfills this need among others.